Have an '85 XJ6. If it sits for a few days the battery is dead. Where do i start?

It really depends on what you are calling "a few days". Your onboard computers, your clock, your radio, and possibly, other electronic components in your car have what is called "keep-alive memory" that draws power from your battery at all times - even with the vehicle shut off. Normal operation of these systems can draw down even a perfectly functioning battery in about 2 weeks or so. If your battery is a little weak, the time that it can keep these sytems running decreases significantly.

The place to start would be to thoroughly test the battery (with the proper equipment and techniques) to ensure that it is in good condition. Then you can have your system tested for an "ignition-off-draw" if you still believe there is a problem. The general rule of thumb is that anything under a 300 milliamp draw is considered "acceptable". Most of the vehicles I have tested usually have a draw of around 125 to 175 milliamps (after all of the computers have gone into sleep mode).

Aftermarket accessories like alarm systems, non-stock radios and remote start systems can add considerably to the "normal" draw that the vehicle was designed for. This is OK as long as the total ignition-off draw remains below 300 milliamps.

If the draw is within the acceptable range and you are allowing it to set for extended periods of time (more than four or five days), you might want to consider a battery maintenance device like a small trickle-charger or a solar panel.

You first have to determine if it is a charging of the battery issue, or the battery itself, or if there is an unintended drain on the battery while sitting.
You can check if the alternator is charging good, by putting a voltmeter on the battery while engine is running. Voltmeter should read over 13 volts, somewhere between about 13.2 volts and tops about 14 volts, or just over. Anything less than 13 volts, and the alternator is not up to par, not charging the battery sufficiently.
If the battery is a little old, and beyond its warranty, it may just be too old to take a full charge, or it might have a dead cell. You can have the battery tested to see if it is still good.
If the alternator and battery are both good and working properly, you can check for an overnight drain with just a simple testlight. Everything in car should be off. Now take off the battery negative terminal and connect the testlight between the negative cable end and the battery negative post. If you have anything draining the battery while it sits, the testlight will light up brightly. If no light, or a very dim bulb, the system is good. A dim light would just be computer and clock memory, a very small draw, like milliamps, and should not be enough to run the battery down.
If you have a bright light on the tester, just start pulling fuses out one at a time, till light goes out. The circuit for the fuse that turned the testlight off is the problem circuit. Something on that circuit should be off and not drawing power, but it is.
Good luck.